Calories in calories out is a fairly safe bet, its not as simple as that but if you nail that youll be pretty much there.
Cutting calories and changing your diet is very hard because you have adapted and have become addicted to a certain calorie consumption as well as probably sugars/bad foods etc.
I see it like quitting smoking or anything else. The first few weeks are the hardest as you rid your system of the crap and get it used to the good.
Start simple, having one less sugar in your tea, swapping a bag of crisps for an apple. most people fail because they go from eating say 3000kals of crisps, buns, mars bars and takeaway to overnight thinking they can eat 2000 cals of salad, lean meat, fruit and veg, yeh right haha you have to have insane will power to do it that way. Change your calorie intake and diet over the course of a month, plan it out, drop this here and there, swap one thing out for another and look at it as a long term commitmet, be patient